15 September 2008

River Douglas shortlist available online

The seven shortlisted designs for the River Douglas Footbridge competition are now available online. This begins a public consultation with the bridge designs displayed locally until 26th September. A winner is due to be announced in mid-October.

I should declare an interest from the outset: I was a competition entrant, and didn't make the shortlist.

One thing of interest is that the competition brief may have been based on essentially a false premise. The Lancashire Remade website implies that the bridge soffit level specified during the competition may be inadequate to allow the river to remain open to navigation. They may therefore be infringing on a legal requirement, and it's unclear whether any of the shortlisted designs could therefore ever be built. I imagine the 110-0dd competitors would have liked to know this before investing their time in the scheme!

Anyway, here are the entries (designers are not identified):

Design 1 is a combination central arch span with two stressed-ribbon side spans. It's nicely detailed, with the two side-by-side structural sections cleverly done. There's one big disadvantage. The bridge will require substantial foundations within the floodplain, which the competition brief describes as peat, sand and clay tidal flat deposits: to the layman, that means sludge. Bedrock is about 20m down. Access to the site is difficult enough anyway (mostly across fields), and access to build the foundations might be a big problem. The abutment foundations outside the floodplain will also be substantial, as they will have to resist considerable forces from the stressed ribbon, which will be only partly balanced by strut members.

Design 2 is, er, a combination central arch span with two stressed-ribbon side spans. This one is more explicit about what they are (they're strictly catenary box girders in design 1), and it's less of an arch, but really it's just a variation on a theme. This one seems better engineered, but the disadvantages are exactly the same. With both the first two designs, it's unclear to what extent the bridges are "landmarks" - one of the brief's criteria was to create a legible marker to the river's crossing point, which suggests that it perhaps ought to be visible from a distance.

Design 3 is definitely the odd-one out, a puzzling steel truss-frame bridge with a short side span and a large Vierendeel-truss main span. I think this is a pretty surprising choice for the shortlist as visually it's quite out-of-place, not even matching the appearance of the old railway bridge that used to sit at this site. Again, there's a foundation in the floodplain with a high thrust component. It seems a bit arbitrary for this scheme.

Design 4 is one I found hard to like, mainly because of its odd proportions. It consists of two asymmetric cable-stays, when one would be easily enough to carry this span, and they are both unnecessarily tall and with exceptionally short back-spans. The net result (again) would be expensive foundations having to carry substantial thrust loads. Not apparent from the elevation, the cables from each mast carry a different edge of the deck, which again just seems unnecessary.

Design 5 brings with it a remarkable sense of deja vu, for yet again it's another arch-and-stress-ribbon combination. This has several interesting components, although ultimately has the same concerns as the similar designs 1 and 2. The designers knows their structural engineering history, hinting at Eladio Dieste as an inspiration with the arch made from a slender concrete shell with brick facing (Dieste would have done it all in pure brick, of course).

If I said at this point that Design 6 combines an arch and stressed ribbon again, you'd be unsurprised. And so it is. Very similar to the first entry in overall conception, with two side-by-side structures incorporating steps on the arch extrados. I find it hard to choose between the four similar designs: Design 5 has the most interesting engineering, but Design 2 looks like it may be the best engineered.

Luckily, Design 7 is something different, with a very Calatravaesque asymmetric cable-stayed bridge, albeit one where the pylons are properly back-stayed (unlike the notirous Alamillo Bridge). I quite like it: it's visually striking, nicely styled, and certainly a landmark. But the foundations will be amongst the most expensive of any of these designs, and it's not really a bridge that suits its context, a very rural location. It's perhaps too bold for such a place.

None of the bridges manage to keep their muddy footprints out of the ecologically sensitive (and constructionally difficult) floodplain, which seems odd since buildability and sensitivity to the environment were two issues raised in the brief. And I'm amazed that four out of seven are of essentially the same structural form: this will severely limit the judge's options at the next stage of the competition, and given the shortcomings of the three other bridges, I expect a stressed-ribbon option to win. I can't tell whether this represents something-in-the-air, a common solution to the site's particular problems, or is just a reflection of a keen liking for this type of bridge by one or more of the judges.

It will be interesting to see which design wins, and also interesting to see after that whether the winner ever gets built. The uncertain legal issues over the navigation are just one issue: this is also a bridge which as yet has no funding in place.

1 comment:

The Happy Pontist said...

Just for the record, here are the designers for each design:

Design 1 - Amin Taha Architects
Design 2 - JDA with Arup
Design 3 - t-hoch-n Architektur
Design 4 - NPS North West Ltd
Design 5 - Guy Nordenson & Associates
Design 6 - Ramboll Whitbybird with Priestman Goode
Design 7 - Nick Hancock Design Studio